1. Field of the Invention
The invention relates to a control device for a process, in which a controlled variable can be influenced with the aid of a plurality of different control elements.
2. Description of the Related Art
In process or method engineering systems, it often occurs that there are two control elements for influencing a controlled variable that complement each another in their effect. Any manipulated variable that is fed to one of the two control elements does not by itself, i.e., without support from the respective other manipulated variable, have enough influence on the process to keep the controlled variable at the set point in all operating states. The two manipulated variables can also be differentiated with respect to the speed with which they affect the process and with respect to the costs and undesired side effects that may be associated with a control action. Normally, both manipulated variables are to be active simultaneously.
An example of this is the regulation of the oxygen partial pressure in a bioreactor, which is frequently referred to as a fermenter. The oxygen partial pressure as a controlled variable, which corresponds to the oxygen content of the broth found in the fermenter, can be influenced both by injecting air or oxygen with the aid of a first control element and by changing the rotational speed of the stirrer by a second control element. The stirrer is to rotate continuously and fresh air is to be supplied on a sustained basis. However, in order to avoid harming the living cells in the reactor, the stirring must not be too vigorous and too much air must not be injected.
One possible approach for regulating such a process would be partial automation in which a conventional Proportional-Integral-Derivative (PID) controller generates a first manipulated variable for one of the two control elements and a second manipulated variable is specified in manual mode. If it is no longer possible to achieve the control objective using the current specified value, an alarm notifies a system operator, who can change the second manipulated variable through manual intervention if required. Within the scope of recipe controls, such interventions in certain phases of batch production can be automated if their necessity in the recipe process can be predicted.
A “split-range control” for solving a somewhat different problem is known from the function manual “SIMATIC Prozessleitsystem PCS 7, PCS 7 Advanced Process Library V71” March 2009, A5E02102721-01 from Siemens AG. With the aid of a “split-range block” behind the controller output, a PID controller is able to distribute its control value to a plurality of different control elements, which act on the same controlled variable using different physical principles and in different directions. A typical example is the temperature control in a reactor that is heated by a live steam valve and cooled by a cooling water valve. Depending on the sign of the control difference, the controller can request heat energy or cooling energy, i.e., is able to work having a bidirectional output, although it is only possible to operate each individual control element in unipolar mode, i.e., in one direction of action. Thus, in the known split-range control, either the one or the other control element is active, but both are not active simultaneously, because they act in different directions. In this example, it makes no sense to heat and cool simultaneously.